Later evidence points strongly to the conclusion that the oculomotor nerve and the premandibular cavity represent not one segment but the fusion of four, while the mandibular cavity represents two segments. In addition to these, Miss Platt has discovered a still more anterior head-cavity, which she has named the anterior cavity, so that the pro-otic segments on this reckoning are seven in number, viz.: (1) the anterior cavity, (2, 3, 4, 5) the premandibular cavity, (6, 7) the mandibular cavity. The somatic muscles belonging to these dorsal segments are the eye-muscles, which are all dorso-ventral in position, and are not the same as the longitudinal somatic muscles, but belong to a distinct dorso-ventral segmental group, the only representative of which at present known in the mesosomatic region is the external rectus innervated by the VIth nerve. These head-cavities, and these muscles of the vertebrate, resemble the corresponding cavities and muscles of the invertebrate to an extraordinary {285}degree, so that it becomes easy to see how the dorso-ventral muscles of the prosomatic segments of the latter have become converted into the eye-musculature of the former. The most powerful proof of all that such a conversion has taken place is that a natural and simple explanation is at once given of the extraordinary course taken by the IVth or trochlear nerve. Ever since neurology began, the course of this nerve has arrested the attention of anatomists. Why should just this one pair of nerve-roots of all those in the whole body be directed dorsalwards instead of ventralwards, and cross each other in the valve of Vieussens, each to supply a simple eye-muscle (the superior oblique) belonging to the other side? For generations anatomists have wondered and found no solution, and yet, without any straining of hypotheses, in consequence simply of the investigation of the anatomy of the corresponding pair of muscles in the scorpion group, the solution is immediately apparent. This pair of muscles alone, of all the musculature attached to the carapace, crosses the mid-dorsal line to be attached to the other side, thus carrying its nerve with it to the other side; by a continuation of the same process the relation of the trochlear to the superior oblique muscle can be explained. The comparison of the eye-muscles of the vertebrate with the dorso-ventral segmented muscles of the invertebrate makes the number and nature of the pro-otic segments much clearer. {286}CHAPTER IX _THE PROSOMATIC SEGMENTS OF AMMOCOETES_